Ring-expansion reaction of cyano-substituted singlet phenyl nitrenes: Theoretical predictions and kinetic results from laser flash photolysis and chemical trapping experiments

On the basis of the open-shell electronic structure of the lowest-singlet s tate of phenylnitrene, it is predicted that substitution of a radical-stabi lizing cyano group at an ortho carbon should facilitate cyclization at that carbon, whereas cyano substitution at the para carbon should retard the ra te of cyclization: These qualitative predictions have been tested computati onally by performing (8/8)CASSCF and CASPT2/6-31G* ab initio calculations a nd experimentally by carrying out laser flash photolysis and chemical trapp ing studies. The calculations and experiments both find that, unlike the ca se with ortho fluoro and ortho methyl substituents, the rate of cyclization at a substituted carbon is not retarded by an ortho cyano group. In contra st, a para cyano group is found, both computationally and experimentally, t o raise the barrier to cyclization of singlet phenylnitrene by >1 kcal/mol.